The present invention relates to a class of testing and measuring meteorological instruments or apparatus that measure naturally occurring solar radiant energy and reflected or emitted terrestrial radiant energy. Specifically, the field of invention is a class of instruments, known as net radiometers, which measure the net difference between incoming solar and outgoing terrestrial radiant energy in the combined short-wave and long-wave far infrared spectral range.
Net radiometers are an important instrument for global climate change research and agro-meteorology. For global climate change research, net radiometers are typically used in glacial studies where they are used to monitor the net loss of energy on an ice sheet. Net radiometers have also been placed on forest floor and above the forest canopy, in combination, to study correlations between biological activity and net energy flux. For agro-meteorology, net radiometers are typically used with a combination of other metrological instruments to measure loss of water in wetlands such as the Everglades or to control irrigation in large farms.
Net radiometers measure the difference between total incoming solar radiant energy and outgoing terrestrial radiant energy. Total incoming solar radiant energy is a combination of incoming short-wave radiant energy from the sun and down-welling long-wave far infrared radiant energy from the sky. Outgoing terrestrial radiant energy is a combination of reflected short-wave radiant energy, and up-welling far infrared radiant energy from the ground.
Typically in the daytime, the majority of net radiant energy contribution comes from incoming short-wave radiant energy from the sun. At night the majority of net radiant energy contribution typically comes from up-welling long-wave far infrared radiant energy from the ground. Short-wave radiant energy is generally defined as radiant energy in the near-ultraviolet, visible, and near-infrared wavelengths. The spectral range is approximately 200 to 4000 nm. Any radiant up-welling or down-welling energy with a spectral range of approximately 4000 nm to 50,000 nm is referred to as long-wave far infrared radiant energy.
Two examples of net radiometer types are four-absorber and two-absorber net radiometers. A two-absorber net radiometer includes a single pair of virtually identical thermal absorbers, one upward facing and the other downward facing. Each thermal absorber is thermally responsive across the short-wave radiant energy and long-wave far infrared radiant energy spectra. The upward facing thermal absorber absorbs radiant energy from the sky while the downward facing thermal absorber absorbs radiant energy either emitted or reflected from the ground.
A four-absorber net radiometer includes two pairs of absorbers. One pair is responsive only to short-wave radiant energy. The other pair is responsive only to long-wave far infrared radiant energy. Each pair has an upward facing and downward facing thermal absorber. While the pairs of absorbers are different from each other, each pair having a different spectral sensitivity, within each pair the absorbers are virtually identical.
Two-absorber net radiometers have a nighttime measurement error up to 13% when compared to their four-absorber counterparts. Information relevant to attempts to address this problem can be found in Cobos, Douglas R. and Baker, John M. Evaluation and Modification of a Domeless Net Radiometer, Agronomy Journal, Vol. 95 (January-February 2003), pp. 180-183. However, that reference does not teach how to solve the problem for a permanent hydrophobic coating nor does it teach the correct coating proportion.
Improvements in the art are always desirable therefore there is a need for a two-absorber net radiometer where the error in nighttime measurement is minimized.